【摘要】 作为温带气旋形成的主要机制，湿斜压不稳定在气旋动热力学研究中占据中心地位，其可进一步分为干斜压不稳定、湿不稳定、非绝热Rossby波和Type C气旋新生（对流层顶干侵入）四类。2016年7月18日黄河气旋快速生成后东移进入华北造成“7·20”特大暴雨，相比气旋成熟期，其初生阶段的动热力机制尚不清楚。本文利用ERA5再分析资料与WRF模式，对该气旋新生过程的湿斜压不稳定机制进行了数值模拟研究。模拟结果指出，对流层中低层呈非绝热Rossby波形态，即系统东移发展主要由垂直运动-非绝热效应的循环所推动，其中波动触发和传播所依赖的垂直运动更多由涡度平流提供；高层位涡汇与非地转风延缓了对流层顶干侵入位涡的东移，维持了高低层相位差，最终在干侵入前部发展出贯穿对流层的位涡柱。利用非线性片段位涡反演，分别从初始场中移除非平衡分量、对流层顶干侵入位涡、低层非绝热源位涡，设计了若干敏感性试验，结合广义垂直运动方程分析可得：本次过程斜压波必须在充足水汽条件下与非绝热过程耦合才能强烈发展，关闭潜热气旋新生将被抑制，干斜压不稳定无法解释本次过程；初始非平衡场的去除不影响本次斜压不稳定性质但将延后系统发展时间，受湿度条件和中尺度环流结构限制，低层非平衡风的活跃区域主要由干斜压动力学控制；该个例近地面位温梯度小，仅依赖低层初始位涡难以有效组织起非绝热Rossby波东传，同时有别于Type C气旋新生，高层位涡异常也不足以激发起强大的中低层非绝热加热。于本次黄河气旋新生而言，一方面要求初始低层位涡异常具有一定强度，以抵消高层干侵入前部伴随的冷却下沉对其的抑制；另一方面也需要高层位涡异常通过垂直渗透以涡度平流形式加强低层位涡东侧上升运动，在高低层初始相位差合适情况下，持续促使非绝热Rossby波东移发展，推动系统进入水汽条件更好的华北地区。干斜压不稳定、非绝热Rossby波和Type C气旋新生机制均不能独立解释本次事件，此次黄河气旋新生是在非绝热Rossby波和对流层顶干侵入混合作用下，初始时刻最优扰动增长形成的。更多还原

【Abstract】 As the main mechanism of extratropical cyclogenesis, moist baroclinic instability plays a central role in the study of cyclone thermodynamics, which can be further divided into four categories: dry baroclinic instability, moist instability, diabatic Rossby wave and Type C cyclogenesis(tropopause intrusion). The ’7·20’ heavy rainstorm was caused by the eastward movement of a Yellow River cyclone into North China after its rapid formation on July 18, 2016. Compared with the mature stage of the cyclone, the mechanism of the initial stage is still unclear. This article uses ERA5 reanalysis data and WRF model to study the moist baroclinic instability of the cyclogenesis event numerically. The results show that mid-lower troposphere presented diabatic Rossby wave pattern, that is, the eastward movement of the system was mainly driven by the cycle of vertical motion and diabatic effect. The vertical motion on which the wave relied was more provided by vorticity advection. The PV sink and the ageostrophic wind in the upper layer delayed the eastward movement of the tropopause intrusion PV, maintaining the phase difference between upper and lower layers. Finally, a PV column formed throughout the troposphere in front of dry intrusion. Using piecewise PV inversion, several sensitivity runs are designed to remove unbalanced circulation, tropopause dry intrusion PV and lower-level diabatic-produced PV from the initial field, respectively. Combined with the analysis of generalized omega equation, it shows that the baroclinic wave in this process must be coupled with the diabatic process with the help of sufficient water vapor to develop strongly. The cyclogenesis was suppressed when the latent heat was turned off. Dry baroclinic instability cannot explain this process. The removal of initial unbalanced field did not affect the baroclinic instability but will delay development of the system. Limited by humidity and mesoscale circulation structure, the active area of lower-level unbalanced flow was controlled by dry baroclinic dynamics. In this case, the gradient of θbottom was too small to organize eastward diabatic Rossby wave by relying only on the initial lower-level PV. Nor can strong lower-level diabatic heating generate as in Type C cyclogenesis by tropopause intrusion. For this Yellow River cyclogenesis case, it is required the initial lower-level PV anomaly to be strong enough to counteract the suppression of the cooling subsidence in front of dry intrusion; on the other hand, it is also required that the dry intrusion, in an appropriate initial phase difference with the lower system, strengthened the ascending motion east of the low-level PV in form of vorticity advection through vertical penetration, so as to promote the eastward momentum of diabatic Rossby wave to enter north China with more saturated environment. None of dry baroclinic instability, diabatic Rossby wave and Type C cyclogenesis could independently explain this cyclogenesis event, which was an initial optimal perturbation growth under the combined effect of diabatic Rossby wave and tropopause dry intrusion.更多还原